How To: Mobility Management

In the US right now, Verizon and T-Mobile are battling it out for who can provide the most extensive LTE coverage. And as of OpenSignal’s 2018 report, the two were in a statistical tie—each was able to provide an LTE signal 93.7% of the time.

These are historic highs for the companies, and objectively impressive numbers, but that still means that they’re failing to provide an LTE signal fully one-twentieth of the time. As such, SRVCC (Single Radio Voice Call Continuity) would still be critical to providing high quality of service (QoS).

Why? Because right now it’s the standard solution for ensuring mid-call handovers from all-IP LTE networks to circuit-switched call bearer 2G/3G networks.

For other operators with lower coverage rates, this functionality is even more critical. Of course, meeting QoS standards for handovers is often easier said than done. Not only does this require network operators to build in time- and environment-sensitive functionality that works for a diverse swath of different devices that mobile subscribers might be using.

It also requires Herculean efforts from test engineers. The 3GPP standards for SRVCC state that the switch from LTE to circuit switched networks should prevent a dropped call 99 percent of the time, which means that no stone can be left unturned when verifying this use case.

The Challenge

When it comes to mobility management, the ability to roll out high quality service is difficult enough on its own. Operators need to arrange for an IRAT handover and a session transfer to happen in parallel at the exact right moment (i.e. as soon as you’ve tracked the subscriber’s handset to the edge of your LTE coverage), regardless of the software or firmware versions at play.

Bandwidth and other resources have to be reserved and allocated in sufficient time to make the handover possible with minimum latency—and it has to work on the first try to prevent the call from dropping. When it comes to service verification, things become even more tricky. This is for two primary reasons.

• Device fragmentation: Each time a new handset is introduced into the marketplace, testers need to ascertain whether or not the existing SRVCC service works with the new firmware specifications.
• Geographic limitations: Because handovers to circuit switched networks only occur when VoLTE is no longer viable, there are only certain areas where this use case can be tested manually.

Both of these hurdles make testing more time consuming, but in markedly different ways. High levels of device fragmentation add to the number of individual test cases in a way that’s certainly not unique to mobility management.

But in an environment in which the average telco tester can only power through 6-10 use cases per day by hand, this can make high test coverage seem impossible. Geographic factors, on the other hand, add an unpredictable variable to the test engineer’s workload.

Essentially, you have to drive test handovers by going out into the field and trying to find the geographic edge of your LTE coverage—i.e. the spot where the handover should happen. Since these spots can be hard to pinpoint, testers can sometimes find themselves driving around for hours seeking out the border of your VoLTE availability.

The Approach

All of the above begs some serious questions about how testers can ensure mobility for their subscribers in a remotely time- and cost-efficient way. As we’ve done before on this blog, we’re going to strongly suggest automation—but what does automation look like for a test case that traditionally involves laborious drive testing?

Essentially, testers will have to recreate the conditions under which an SRVCC handover would typically occur, e.g. a waning LTE signal. To that end, testers could use an RF switch matrix with which to control the signal strength to the device.

In this way, you can simulate the waning packet switched service that would indicate the need for a handover to circuit switched, and from there you can verify that the IRAT handover and the session transfer are actually triggered and that the call is connected within the acceptable latency limits (0.3 seconds of voice interruption, as per the 3GPP).

Again, there’s no guarantee that this service will work seamlessly on every individual device just because it worked on others that have already been tested. Thus, it’s crucial to incorporate the RF switch matrix and the relevant end user devices into your overall automation framework.

With SEGRON’s ATF (automated test framework), this would mean controlling all of the devices from a centralized interface, and executing the relevant test scripts as needed. This could then be repeated as needed for new devices entering the marketplace or to verify service after making other changes to your network.

How Effective Handovers Impact ROI

Like we talked about above, drive testing handovers can quickly become a cumbersome and time-consuming process. As such, automating verification for this use case in such a way as to remove driving time and save manual effort in the testing itself stands to speed things up considerably.

Using SEGRON’s ATF, the automation framework would empower you to run through hundreds of test cases per instance per day. Instead of being a tremendous burden in terms of engineer resources, this test case can become just another spoke in your regular test flow.

Time is money, so this puts you in a position to save costs. By the same token, it also puts you in a position to speed up time-to-market. As new handsets come onto the market, for instance, you’ll be able to verify handover functionality—or find and address the root causes of any bugs—in record time.

This will delight dealers and end-users alike, and stands to have a positive impact on your subscriber base. Likewise, because improved testing in this area can help you to raise your network quality standards overall, automating mobility management verification gives you a real chance to boost your perceived QoS.

The end result? Reduced subscriber churn, leading to improved margins and enhanced profitability—even if your LTE network already covers 19 out of 20 calls.